The present invention relates to measuring height variations of a surface of a three-dimensional object. More specifically, the present invention relates to phase profilometry systems.
Mapping the surface of a three-dimensional object has many applications in a wide variety of fields. Such a determination can be used to identify an object, the position of an object, and the size and shape of an object. This data may be used as an input to an automated system.
There are a number of, non-contact techniques which can be used to determine a surface profile. For example, U.S. Pat. Nos. 4,641,972 and 5,646,733 describe systems in which a fringe pattern is projected onto a surface of an object. By measuring the phase of the fringe pattern on the object, the surface profile can be determined. In general, such systems project a fringe pattern onto the object at a plurality of phases, and capture images of the object at each of the those phases. The phase displacement can be obtained through physical translation of the object or through manipulation of the projection optics used to generate the fringe pattern.
When such systems are used in automation, for example, it is particularly desirable for them to operate at a high speed and with a high degree of reliability, both in measurement accuracy and measurement repeatability. These objectives can be particularly difficult to achieve when relatively small feature sizes are being mapped. As used here, the term repeatability refers to variations in the measurement of an object""s height, volume or area when it is presented to the system at different angles and at different positions within the field of view of the instrument. The speed of the system refers to the inspected area per unit time. The speed of a system, in general, can be increased by sacrificing the repeatability.
An optical system for computing a height of a target on a surface includes a light projector for projecting light. The light passes through a patterned reticle and a projector lens so as to illuminate the target with an image of the pattern that varies sinusoidally in intensity. The light is projected telecentrically between the reticle and the projector lens, and a camera is positioned along a receive optical path. The camera receives an image of the target through a receive lens. The target and the pattern move at least three times with respect to each other, and the camera acquires an image of the object at each of the positions. A processor receives the images and computes the heights accordingly.
In the preferred embodiment of the invention, the lens in the projector is telecentric both on the reticle and the target side, and the camera lens is telecentric on the target side. Another embodiment includes matched vertical pupil height in the projector and the receive optical path, so as to further increase the repeatability without adding computational complexity. In a final embodiment, a rotating refractor in positioned between the reticle and the projector lens.